The Labour Party was targeted in a “large-scale and sophisticated” cyber attack designed to take its systems “entirely offline”.
The “full functionality” of Labour’s online campaign systems was “reduced” as a result, the party said.
Officials have reported the attack to the National Cyber Security Centre.
A security expert said this kind of attack involved “bombarding websites or digital services with large numbers of traffic until they collapse”.
Labour Party leader Jeremy Corbyn said this afternoon that he is “very nervous” following the attack.
He told reporters: “It was a very serious attack against us, as far as we’re aware none of our information was downloaded.”
And he added: “If this is a sign of things to come in this election, I feel very nervous about it all because a cyber attack in a general election is very suspicious.”
He said an attack against the NHS in 2017 had been “incredibly dangerous”.
Mr Corbyn said the party was “looking into” who was responsible for the attack.
A Labour spokeswoman said the attempt failed because of the party’s “robust security systems” and officials are confident that no data breach occurred.
There is no indication yet about who was responsible.
Niall Sookoo, the party’s executive director for elections and campaigns, said in a letter to colleagues: “Yesterday afternoon our security systems identified that, in a very short space of time, there were large-scale and sophisticated attacks on Labour Party platforms which had the intention of taking our systems entirely offline.
Get blogs, cyber securityy industry news, updates, and articles delivered right to your inbox. We email once a week with curated topics just for you. No spam, just fun.